Overlay Measurement on Double Patterning Substrate

ABSTRACT

A method of measuring overlay between a first structure and a second structure on a substrate is provided. The structures include equidistant elements, such as parallel lines, wherein the equidistant elements of the first and second structure alternate. A design width CD 1  of the elements of the first structure is different from a design width CD 2  of the elements of the second structure. The difference in design width can be used to identify measurement points having incorrectly measured overlay errors.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of the filing date of U.S.Provisional Application 60/960,698 filed on Oct. 10, 2007. The subjectmatter of that application is incorporated herein by reference as iffully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of measuring overlay on asubstrate including two structures manufactured by way of a doublepatterning technique. It also relates to a method of manufacturing twostructures on a substrate which can be used to measure overlay.

2. Related Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

To improve the critical dimension (CD) of modem semiconductors, atechnique called Double Patterning (DP) is used. In this technique twoexposures are executed for one single layer on the substrate. For eachexposure a specific mask is used that together with the other mask formsthe desired pattern. Instead of Double Patterning, so-called MultiplePatterning is used wherein more than two exposures are used to form thedesired pattern in a layer on a substrate.

Double patterning is used with critical dimensions of less than 100 nm.When forming a second structure next to a first structure in order toform a desired pattern, an overlay error may be present that is definedas the difference between the actual position of the second structurerelative to the desired position of the second structure. The overlayerror occurs due to alignment issues and/or optical errors in alithographic apparatus. The overlay error will depend on the position ona substrate. In case of double patterning, the overlay error is referredto as Double Patterning Overlay error, i.e. DPTO error. To indicate thatthis error may vary as a function of the position on a substrate, belowthe term DPTO(x,y) is used.

So-called CDU wafers (CDU-critical dimension uniformity) are used tomeasure the DPTO(x,y) of a system. A CDU wafer is measured using forexample a Scanning Electron Microscope (SEM) to obtain images of the CDUwafer at multiple points on the substrate. The CDU wafer may for examplecomprise a pattern of parallel lines that is formed by a doublepatterning technique, where elements (i.e. lines) of a first structurealternate with elements of a second structure. The first structure isformed using a first exposure and the second structure is formed at asecond exposure. The images can be made around predetermined points(x_(i), y_(i)) on the substrate. The images are input for an imageprocessor that is arranged to measure the DPTO for each image. To dothis, the image processor needs to identify the first structure from thesecond structure. A possible solution is to instruct the SEM to scanareas around the predetermined points (x_(i),y_(i)) so that thepredetermined points (x_(i), y_(i)) are positioned at a predefinedposition on each image (for example the exact centre of the image).Knowing that one of the lines of the first structure (or the secondstructure) is positioned at exactly position (x_(i), y_(i)), it can beconcluded by the image processor that the line in the centre of theimage is part of the first structure. This information is needed tocalculate the correct DPTO(x,y). However, due limitations in the hard-and software of the metrology tool, the identification of structuresmight be unreliable, resulting in errors of the calculated overlay errorDPTO(x,y).

BRIEF SUMMARY OF THE INVENTION

It is desirable to improve the reliability of the calculated doublepatterning overlay error on a substrate having test structures.

According to an aspect of the invention, there is provided a method ofmeasuring overlay between a first structure and a second structure on asubstrate. The first structure includes a first plurality of equidistantelements and the second structure includes a second plurality ofequidistant elements. The first plurality of equidistant elementsalternate with the second plurality of equidistant elements, and whereina design width CD₁ of the first plurality of equidistant elements isdifferent from a design width CD₂ of the second plurality of equidistantelements. The method includes receiving a plurality of points ofinterest (x_(i),y_(i)) to be investigated on the substrate. An overlayerror DPTO is determined between the first structure and the secondstructure for each point of interest (x_(i),y_(i)). For each point ofinterest (x_(i),y_(i)) there is measured a width W₁ of a first elementand a width W₂ of a second element. For each point of interest(x_(i),y_(i)) a difference is determined between the measured width W₁of the first element and the width W₂ of the second element. Adistribution is determined of the number of points of interest havingapproximately the same value for the difference as a function of thedifference. If the distribution has a sub-distribution around a valueW₁-W₂ having an opposite sign as compared to the difference CD₁-CD₂,points of interest are identified that are associated with thesubdistribution as having incorrectly measured overlay errors.

According to an aspect of the invention, there is provided a method ofmanufacturing an overlay measurement structure on a substrate. Themethod includes forming a first structure on the substrate using a firstexposure of a first patterning device, the first structure having afirst plurality of equidistant elements. A second structure is formed onthe substrate using a first exposure of a second patterning device. Thesecond structure has a second plurality of equidistant elements. Thefirst plurality of equidistant elements alternate with the secondplurality of equidistant elements, and a design width of the firstplurality of equidistant elements is different from a design width ofthe second plurality of equidistant elements.

According to an aspect of the invention, there is provided an overlaymeasurement structure manufactured according to the method as describedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus which can be used to carry outthe manufacturing method according to an aspect of the invention;

FIG. 2 schematically shows an image of a part of a CDU wafer accordingto the state of the art;

FIG. 3 shows an example of a configuration of structures according to anembodiment of the invention;

FIG. 4 diagrammatically shows part of system arranged to measure overlayerrors on a substrate;

FIG. 5 schematically shows part of two structures having an overlayerror;

FIG. 6 is a graph showing a distribution of measurement point as afunction of the difference in width of the elements of the twostructures;

FIG. 7 shows an example of a configuration of structures according to anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically depicts a lithographic apparatus which can be usedto carry out the manufacturing method according to an aspect of theinvention. The apparatus includes an illumination system (illuminator)IL configured to condition a radiation beam B (e.g. UV radiation). Asupport structure (e.g. a mask table) MT is constructed and arranged tosupport a patterning device (e.g. a mask) MA and is connected to a firstpositioner PM configured to accurately position the patterning device inaccordance with certain parameters. A substrate table (e.g. a wafertable) WT is constructed and arranged to hold a substrate (e.g. aresist-coated wafer) W and is connected to a second positioner PWconfigured to accurately position the substrate in accordance withcertain parameters. A projection system (e.g. a refractive projectionlens system) PS is configured to project a pattern imparted to theradiation beam B by patterning device MA onto a target portion C (e.g.comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. The pattern imparted to the radiationbeam may not exactly correspond to the desired pattern in the targetportion of the substrate, for example if the pattern includesphase-shifting features or so called assist features. Generally, thepattern imparted to the radiation beam will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Illuminator IL receives a radiation beam from a radiation source SO. Thesource and the lithographic apparatus may be separate entities, forexample when the source is an excimer laser. In such cases, the sourceis not considered to form part of the lithographic apparatus and theradiation beam is passed from the source SO to the illuminator IL withthe aid of a beam delivery system BD comprising, for example, suitabledirecting mirrors and/or a beam expander. In other cases the source maybe an integral part of the lithographic apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam B, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 schematically shows an image 20 of a part of a CDU waferaccording to the state of the art. This image could have been made usinga SEM or any other suitable imaging apparatus. The image 20 comprises aplurality of lines 21, 23, 25 of a first structure produced during afirst exposure and a plurality of lines 22, 24 of a second structureproduced during a second exposure. In order to calculate the local DPTO,i.e. the DPTO at the point (x_(i),y_(i)) around which the image of FIG.2 is made, the image is processed by an image processor. The imageprocessor needs to distinguish structure 1 from structure 2. Due toinaccuracies of the SEM, it can be difficult to distinguish structure 1from structure 2.

Therefore, according to an embodiment of the invention, a substrate isused for calculating the DPTO which comprises a first structure and asecond structure, wherein the first structure comprises a firstplurality of equidistant elements and the second structure comprises asecond plurality of equidistant elements, wherein the first plurality ofequidistant elements alternate with the second plurality of equidistantelements, and wherein a design width CD₁ of the first plurality ofequidistant elements is different from a design width CD₂ of the secondplurality of equidistant elements. An example structures is shown inFIG. 3.

FIG. 3 shows an image 30 having parallel lines 31, 33 produced by way ofa first exposure, and parallel lines 32, 34 produced by way of a secondexposure. The width CD1 of the lines 31, 33 is unequal to the width CD2of the lines 32, 34. In this case the width CD1 is larger than the widthCD2. Typical values for CD1 are between 40-70 nm and typical values forCD2 are between 20-50 nm.

FIG. 4 diagrammatically shows a SEM 40 that is arranged to scanpredefined parts of a CDU substrate 41. The SEM 40 receives datarepresenting points (x,y) to be investigated on the CDU substrate 41.The SEM 40 produces images 42 of for example areas around each of thepoints (x_(i),y_(i)) to be investigated in which the point (x,y) are inthe centre of the images. It should be clear to the skilled person thatthe SEM 40 may be arranged to produce images that are determined by thepoints (x,y) in any other way. An image Ii could for example be made ofa rectangular area a corner of which is the point (x_(i),y_(i)). Theimages 42 are input for a image processor 43 which is arranged tocalculate the DPTO(x,y) for each point (x_(i),y_(i)) to be investigated.This DPTO(x,y) is input for a data processor 44.

It is noted that instead of using a SEM, another metrology tool could beused, such as a scatterometer, to measure areas determined by the points(x_(i),y_(i)), as will be clear to the skilled person.

In FIG. 5, an example is given of how the image processor 43 candetermine a DPTO using a specific image. FIG. 5 shows only the relevantlines of the image. A first structure comprises lines 51, 53 the widthof which is larger than the width of lines 52, 54 of a second structure.In FIG. 5, the distance between the left side of the line 53 and theleft side of the line 52 is referred to as P_(L). The distance betweenthe right side of the line 53 and the right side of the line 52 isreferred to as PR. Furthermore, a pitch P is depicted being half thedistance between the centre of the first structure lines 51, 53. Typicalvalues for the design pitch P on a CDU substrate are around 64 nm. Adifference P₁ between the centre of line 52 and the centre of line 53 isreferred to as P₁. This P₁ pitch can be calculated as follows:

P ₁=(P _(L) −P _(R))/2  (1)

Now the DPTO can be calculated as follows:

DPTO=P−P ₁  (2)

The DPTO is calculated at a plurality of measurement points on the CDUsubstrate so that a (non)uniformity of the overlay error can bedetermined. So for each point (x_(i),y_(i)) a value DPTO(x_(i),y_(i)) isdetermined. According to an embodiment, for each measurement point(x_(i),y_(i)) a width W₁ of the first structure line 53 is measured andalso a width W₂ of the second structure line 52, see FIG. 5. Adifference between W1 and W2 for each measurement point passed to thedata processor 44. The data processor is arranged to determined adistribution shown in FIG. 6.

In FIG. 6, the number of measurement points having a value approximatelyequal to W₁-W₂ is shown as a function of W₁-W₂. As can be seen from FIG.6, a relative large distribution of measurement points is present aroundthe design values CD₁-CD₂ resulting from the fact that actually thegraph of FIG. 6 is a probability function around an expected valueCD₁-CD₂ with variations due to inaccuracies of the manufacturing of thelines and the inaccuracies of measuring the widths of the lines.However, as can be seen in FIG. 6, at the left side of there is also asecond (smaller) distribution. This distribution (also referred to assubdistribution) has arisen around a value CD₂-CD₁ which has an oppositesign of the value CD₁-CD₂. The second subdistribution is the result ofan incorrect identification of the lines by the image processor 43. Thisincorrect identification may be resulting from an inaccurate positioningof the substrate in the SEM so that the image processor, when lookingfor the line 53 of the first structure, actually finds a line 54, seeFIG. 5 belonging to the second structure having a smaller width. Thiswill result in an incorrect DPTO(x_(i),y_(i)). In fact, the incorrectDPTO(x_(i),y_(i)) may be corrected by switching the sign of theincorrect DPTO error of the measurement point in the smallerdistribution of FIG. 6 resulting in the correct DPTO error.

Please note that the two sub-distributions of FIG. 6 may have arbitraryrelative dimensions. Both distributions may be substantially equal inheight and even the left subdistribution (representing the incorrectmeasurement points) can be larger than the right distribution. In theextreme case, the left distribution can be the only distributionpresent. In the latter case, the DPTO is incorrectly measured in allpoints. Of course, also the right distribution can be the only onepresent resulting from the fact that all DPTO errors are measuredcorrectly.

Although the structures discussed thus far in the embodiments consist oflines, the two structures manufactured on the substrate may havedifferent configurations. An example of another embodiment is shown inFIG. 7. There, another possible configuration of the two structures isshown wherein the relatively large circular elements 70, 71, 72, 73, 74alternate with relatively small circular elements 75, 76, 77. Anadvantage of this configuration is that it can be used to determine aoverlay error in two directions, such as an X-direction and aY-direction. In FIG. 7, the relevant parameters are indicated which canbe input for the following formula's in order to produce the DPTO_(X)and the DPTO_(Y):

DPTO_(X) =P _(X)−(P _(Lx) +P _(Rx))/2, with P_(X) the design pitch inthe X-direction

DPTO_(Y) =P _(Y)−(P _(Ly) +P _(Ry))/2, with P_(Y) the design pitch inthe Y-direction.

Similar to the method described above with reference to FIG. 5 and FIG.6, an error due to the SEM inaccuracies can be corrected for. In thiscase, two distributions for each image (i.e. measurement point) need tobe calculated.

The elements of the structures (e.g. the lines) can be manufactured asholes in a top layer of the substrate. Alternatively, they can bemanufactured as elevations on a top layer, or the elements of the firststructure could be holes and the elements of the second structure couldbe elevations. Other configurations are possible as will be appreciatedby the skilled person.

The different widths CD₁ and CD₂ of the structures on the substrate canbe achieved using two masks MA₁ and MA₂ wherein a first mask MA₁comprises elements, such as lines, having a width CD₁ and a second maskMA₂ comprises elements having a width CD₂. Alternatively, only one maskMA may be used that comprises only the first structure, such as theparallel lines 31, 33 of FIG. 3, and that this mask MA is exposed twiceusing a different dose and a different alignment. In that case, only onemask is needed to manufacture the two structures.

The method of measuring overlay as described above can be used as aqualification test. In this case, the substrate may be a CDU wafercomprising structures having alternating elements in which thestructures are the main feature on the reticle. Alternatively, thestructures may be test features on a product wafer. In that case thestructure could be manufactured in the scribe-lines on the substrate.

Measuring overlay on double patterning wafers, may be performed onresolution of the lithographic apparatus. So, in an embodiment, thewidth of the first plurality of equidistant elements is less than 50 nm.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1-11. (canceled)
 12. A method of measuring overlay between a firststructure and a second structure on a substrate, said first structurecomprising a first plurality of equidistant elements and said secondstructure comprising a second plurality of equidistant elements, whereinsaid first plurality of equidistant elements alternate with said secondplurality of equidistant elements, and wherein a design width CD₁ ofsaid first plurality of equidistant elements is different from a designwidth CD₂ of said second plurality of equidistant elements, said methodcomprising: receiving a plurality of points of interest (x_(i),y_(i)) tobe investigated on said substrate; determining an overlay error DPTObetween said first structure and said second structure for each point ofinterest (x_(i),y_(i)); measuring for each point of interest(x_(i),y_(i)) a width W₁ of a first element and a width W₂ of a secondelement; determining for each point of interest (x_(i),y_(i)) adifference between said measured width W₁ of said first element and saidwidth W₂ of said second element; determining a distribution of thenumber of points of interest having approximately the same value forsaid difference as a function of said difference; if said distributioncomprises a sub-distribution around a value W₁-W₂ having an oppositesign as compared to the difference CD₁-CD₂, identifying points ofinterest associated with said sub-distribution as having incorrectlymeasured overlay errors.
 13. A method of measuring overlay according toclaim 12, further comprising: switching signs of said incorrectlymeasured overlay errors to render correct overlay errors.
 14. A methodof measuring overlay according to claim 12, wherein said overlay errorDPTO is determined by: determining a distance PL between a first edge ofsaid first element and a corresponding first edge of said second elementpositioned next to said first element in a first predefined direction;determining a distance PR between a second edge of said first elementand a corresponding second edge of said second element, said second edgebeing situated opposite said first edge and said corresponding secondedge being situated opposite said corresponding first edge; calculatingsaid overlay error DPTO for each point of interest (x_(i),y_(i)) usingformula:DPTO=P−(P _(L) +P _(R))/2, with P being a design pitch;
 15. A method ofmeasuring overlay according to claim 13, wherein said overlay error DPTOis determined by: determining a distance PL between a first edge of saidfirst element and a corresponding first edge of said second elementpositioned next to said first element in a first predefined direction;determining a distance PR between a second edge of said first elementand a corresponding second edge of said second element, said second edgebeing situated opposite said first edge and said corresponding secondedge being situated opposite said corresponding first edge; calculatingsaid overlay error DPTO for each point of interest (x_(i),y_(i)) usingformula:DPTO=P−(P _(L) +P _(R))/2, with P being a design pitch;
 16. A method ofmeasuring overlay according to claim 12, wherein said first element islocalized as being an element present at or closest to said point ofinterest (x_(i),y_(i)).
 17. A method of measuring overlay according toclaim 12, wherein said DPTO for said plurality of points of interest(x_(i),y_(i)) is determined by means of making an image I_(i) of an areadetermined by each of said plurality of points of interest (x_(i),y_(i))using a Scanning Electron Microscope.
 18. A method of measuring overlayaccording to claim 12, wherein said first plurality of equidistantelements comprises a first plurality of parallel lines, and wherein saidsecond plurality of equidistant elements comprises a second plurality ofparallel lines.
 19. A method of measuring overlay according to claim 12,wherein said first plurality of equidistant elements comprises a firstplurality of substantially circular elements, and wherein said secondplurality of equidistant elements comprises a second plurality ofsubstantially circular elements.
 20. A method of measuring overlayaccording to claim 12, wherein said width of said first plurality ofequidistant elements is less than 50 nm.
 21. A method of manufacturingan overlay measurement structure on a substrate, said method comprising:forming a first structure on said substrate using a first exposure of afirst patterning device, said first structure comprising a firstplurality of equidistant elements; forming a second structure on saidsubstrate using a first exposure of a second patterning device, saidsecond structure comprising a second plurality of equidistant elements,wherein said first plurality of equidistant elements alternate with saidsecond plurality of equidistant elements, and wherein a design width ofsaid first plurality of equidistant elements is different from a designwidth of said second plurality of equidistant elements.
 22. An overlaymeasurement structure manufactured according to a method comprising:forming a first structure on said substrate using a first exposure of afirst patterning device, said first structure comprising a firstplurality of equidistant elements; forming a second structure on saidsubstrate using a first exposure of a second patterning device, saidsecond structure comprising a second plurality of equidistant elements,wherein said first plurality of equidistant elements alternate with saidsecond plurality of equidistant elements, and wherein a design width ofsaid first plurality of equidistant elements is different from a designwidth of said second plurality of equidistant elements.
 23. (canceled)